<!DOCTYPE html>
<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
#target {
  --static: 100;
}
</style>
<div id="target"></div>
<script>
setup(() => {
  for (let name of ['--test', '--static', '--animated']) {
    CSS.registerProperty({
      name,
      syntax: '<number>',
      initialValue: '0',
      inherits: false,
    });
  }
});

test(() => {
  // --test is animating from a static value to an animated value resulting in it changing quadratically.
  let animation = target.animate({'--test': ['var(--static)', 'var(--animated)']}, 100);
  let referencedAnimation = target.animate({'--animated': ['200', '300']}, 100);

  referencedAnimation.currentTime = 25;
  {
    animation.currentTime = 25;
    // lerp(100, lerp(200, 300, 25%), 25%) == 0.75*100 + 0.25*(0.75*200 + 0.25*300) == 131.25
    assert_equals(getComputedStyle(target).getPropertyValue('--test'), '131.25', 'target at 25%, to at 25%');

    animation.currentTime = 75;
    // lerp(100, lerp(200, 300, 25%), 75%) == 0.25*100 + 0.75*(0.75*200 + 0.25*300) == 193.75
    assert_equals(getComputedStyle(target).getPropertyValue('--test'), '193.75', 'target at 75%, to at 25%');
  }

  referencedAnimation.currentTime = 75;
  {
    animation.currentTime = 25;
    // lerp(100, lerp(200, 300, 75%), 25%) == 0.75*100 + 0.25*(0.25*200 + 0.75*300) == 143.75
    assert_equals(getComputedStyle(target).getPropertyValue('--test'), '143.75', 'target at 25%, to at 25%');

    animation.currentTime = 75;
    // lerp(100, lerp(200, 300, 75%), 75%) == 0.25*100 + 0.75*(0.25*200 + 0.75*300) == 231.25
    assert_equals(getComputedStyle(target).getPropertyValue('--test'), '231.25', 'target at 75%, to at 25%');
  }
}, 'Animated registered custom properties can var() reference other animated registered custom properties across separate Animations.');
</script>
